1. Field of the Invention
The present invention relates to a method for producing a copolymer for a cement admixture, a cement admixture containing the copolymer, and a cement composition containing the cement admixture. More specifically, the present invention relates to a method for producing a copolymer for a cement admixture which can be advantageously used in a cement composition such as cement paste, mortar and concrete, a cement admixture containing the copolymer and a cement composition containing such a cement admixture and widely used to build civil engineering and construction structures, etc.
2. Description of the Related Art
A cement admixture has been widely used as a water reducing agent for a cement composition such as cement paste, mortar and concrete, and is now an indispensable material to build civil engineering and construction structures, and the like from a cement composition. Such a cement admixture serves to improve strength and durability of hardened material by enhancing fluidity of a cement composition to reduce water content of a cement composition. Heretofore, naphthalene-based admixtures such as condensates of naphthalene sulfonic acid and formalin, melamine-based admixtures such as condensates of melamine sulfonic acid and formalin, polycarboxylic acid-based admixtures such as aqueous vinyl copolymers of a polyalkylene glycol monoester-based monomer and a (meth)acrylic acid-based monomer and/or a dicarboxylic acid-based monomer have been known. Among these water reducing agents, since a polycarboxylic acid-based polymer fulfills higher water reducing performance as compared with conventional water reducing agents such as naphthalene-based water reducing agent, in particular, a cement admixture containing this polycarboxylic acid-based polymer has had many actual results as a high performance AE water reducing agent.
In such a cement admixture, in addition to the water reducing performance for a cement composition, such a cement admixture is required that can impart good viscosity thereto, in consideration of workability at a field where a cement composition is used. In general, a cement admixture used as a water reducing agent exhibits superior water reducing performance by lowering viscosity of a cement composition. In addition to such water reducing performance, in order to make the work easy at a field where a cement composition is used, it is required to maintain a certain degree of viscosity at a construction site of civil engineering and construction structures, etc. With a cement admixture fulfilling such performance, work efficiency can be improved in building of civil engineering and construction structures, etc.
Aiming at the objects, various cement admixtures have been proposed. For example, JP-A-2004-182583 has disclosed a cement admixture essentially containing a ternary system of or a more multi-component system of a copolymer obtained by polymerizing monomer components comprising 3 or more kinds of monomers including an ethylene-based monomer (A) having a polyoxyalkylene group, an unsaturated organic acid-based monomer (B) and other unsaturated monomers (C), wherein molar ratios of an ethylene-based monomer (A) having a polyoxyalkylene group and other unsaturated monomers (C) in the copolymer are changed during the polymerization. Also, JP-A-2001-180998 has disclosed a binary or a ternary system of cement admixture which can fulfill superior dispersibility and fluidity. The cement admixture disclosed therein is a concrete admixture containing a copolymer mixture obtained by copolymerizing at least one monomer (A) represented by the general formula (a):
(wherein R1 and R2 represents a hydrogen atom or a methyl group; m is in the range of 0 to 2; R3 represents a hydrogen atom or —COO(AO)nX; p is 0 or 1; AO represents an oxyalkylene group with carbon atoms of 2 to 4 or an oxystyrene group; n is in the range of 2 to 300; and X represents a hydrogen atom or an alkyl group with carbon atoms of 1 to 18), and at least one monomer (B) represented by the general formula (b):
(wherein R4 to R6 represent a hydrogen atom, a methyl group or (CH2)m1COOM2, wherein (CH2)m1COOM2 may form an anhydride together with COOM1 or another (CH2)m1COOM2, provided that M1 and M2 of these groups are not present; M1 and M2 represent a hydrogen atom, an alkali metal, an alkaline earth metal, an ammonium group, an alkylammonium group or a substituted alkylammonium group; and m1 is in the range of 0 to 2), wherein the molar ratio of the monomers (A) and (B), (A)/(B) ratio, is changed at least one time during the reaction.
Furthermore, for the purpose to enhance performance of such a cement admixture, it is necessary that raw materials to be used should be severely controlled. For example, JP-A-11-71151 has disclosed a method for producing polycarboxylic acid for a cement admixture obtained by subjecting polyalkylene glycol having a peroxide value of not higher than 0.7 meq/kg to esterification reaction with (meth)acrylic acid to obtain polyalkylene glycol(meth)acrylate, and then copolymerizing the resultant polyalkylene glycol(meth)acrylate with (meth)acrylic acid.